Welcome to the community, @Jess_Databases!
Wow, I didn’t know about this limitation of the SendGrid app. That is an extremely unfortunate limitation of the SendGrid app, and it’s a real shame that you had to discover it on your own through trial-and-error instead of Airtable taking the time to properly document the limitations of their own app! I would highly recommend that you email email@example.com to make them aware of this limitation, and at the very least, Airtable should update their documentation for this app.
But anyways, welcome to the Wild West of using Airtable, where the users get to discover Airtable’s limitations on our own!
It seems like your only workaround for using SendGrid with more than 8,000 characters would be to send your emails directly using SendGrid’s API. This is unfortunate because you’ll have to rebuild your workflow, but the blessing in disguise here is that it will give you a ton more flexibility, because you can design fully professional HTML templates in SendGrid, and then send your Airtable data via your official SendGrid templates. Another blessing in disguise here is that you’ll be able to automate the sending of your emails as well (if you’d like), because Airtable apps can’t currently be automated.
If you do go down the Integromat path, you’ll have to spend a little bit of time learning the Integromat platform, along with re-envisioning how your email sending automation will get triggered by Airtable. But once you get through this small learning curve, you’ll really appreciate the power & flexibility & ease of use of Integromat!
p.s. If you have a budget for this project and you need to hire an expert Airtable & Integromat developer to help you set this up, please feel free to send me a private message or contact me through my website at scottworld.com.